Mutability vs immutability describe the relationship between identity and state. Mutable values retain identity when their state changes. For immutable values,

Mutability vs immutability describe the relationship between identity and state. Mutable values retain identity when their state changes. For immutable values, getting a new state requires a new identity, and importantly, existing identities never have their states changed. Variables can be described by types. That is, a mutable slot referencing a value can be a value itself (a reference to a reference) and we can use subtype relationships to describe it. Variables are covariant when used as inputs (i.e code reads from the variable) and contravariant when used as outputs, and invariant when used as both. You can logically supply a Box<Cat> to a vet(in Box<Animal>) routine, and supply Box<Animal> to catchAndStore(out Box<Cat>). Substitute `ref X` for `Box<X>` when using a language that can pass variables by reference.